class Solution {
    public String convert(String s, int numRows) {
        if(numRows == 1) return s;
        int n = s.length();
        StringBuilder[] sb = new StringBuilder[numRows];
        int row = 0;
        int dir = 1;

        for(int i = 0; i < n; i++) {
            if(sb[row] == null) sb[row] = new StringBuilder();
            sb[row].append(s.charAt(i));
            if(row == numRows - 1 || ((row == 0) && (i != 0))) dir = -dir;
            row += dir;
        }

        StringBuilder result = new StringBuilder();
        for(int i = 0; i < numRows; i++) {
            if(sb[i] == null) continue;
            result.append(sb[i]);
        }
        return result.toString();
    }
}